With your disign the only thing that is being suspended is the seat, so has no real advantage over a suspention seatpost. Most who ride really knarly terrain preffer a fat 3" tire. Those who do use a suspention seatpost usually just ride XC.
There was a thread recently where a guy made his own design of a full suspention uni (hub, frame).
There was a thread hear a while ago discussing all sorts of design possibilities, all having various drawbacks. The biggest being drive control, strength, excess weight, complexity and therefore cost.